1. Field of the Invention
This invention relates to removing organic contaminants from waste water streams. More particularly, this invention relates to pretreating a waste water stream to remove suspended inorganic solids and suspended organic contaminants generally described as oil and, thereafter, passing the pretreated waste water through the bed(s) of activated carbon. In particular, this invention relates to an improved process for employing activated carbon treatment of pretreated waste water streams by adding a controlled amount of oxygen to the activated carbon bed(s) in the range of from about 0.05 to about 0.15 pound of oxygen consumed per pound of total COD contaminants removed from the waste water. Preferably, the oxygen is dissolved in the waste water prior to contacting the bed(s) of activated carbon. The addition of this critical amount of oxygen consumed within the activated carbon bed(s) provides a balance between anaerobic and aerobic biological degradation of the contaminants adsorbed on the activated carbon so as to suppress the evolution of hydrogen sulfide from the bed(s) while at the same time minimizing the formation of aerobic sludge produced by the biological oxidation occurring in the bed(s). The biological activity in the activated carbon bed(s) provides for a substantial increase in the effective organic contaminant removal capacity of the activated carbon.
2. DESCRIPTION OF THE PRIOR ART
The most common method employed for removing impurities from waste water comprises a primary settling step wherein a major portion of the solids suspended therein is removed with or without the aid of chemical flocculating agents. A secondary treat step may then be performed to decompose by bacteriological action the remaining suspended solids which are usually present in a concentration ranging from about 50 to about 150 ppm. In addition, the secondary treating step generally employs vigorous aeration in order to further decompose the dissolved organic materials by bacterial action. The effluent from this secondary biological treating step is settled to remove the bacteria as a sludge, at least a portion of which is then recycled to the secondary treating zone. The clarified effluent is passed into rivers or streams, generally with no further purification.
This method of biologically treating waste water was developed primarily for the treatment of sanitary or household sewerage, which is typically found in a municipal sewer, and has generally worked quite satisfactorily. Recently, however, industrial plants have been discharging waste waters in municipal sewer systems. This has resulted in serious difficulties since industrial waste waters also contain a significant amount of nonbiodegradable pollutants and toxic materials. These nonbiodegradable pollutants are not removed from the industrial waste waters and the toxic materials therein often destroy the bacteria in the secondary biological treatment step, thereby rendering the treatment plant inoperable for a period of time while the toxic materials are purged from the system and new bacterial growth reestablished. In addition, conventional biological oxidation has not demonstrated the ability to produce consistently high quality effluent when treating waste waters from petroleum refining and petrochemical manufacturing operations.
In determining the degree of contamination of a waste water stream, certain recognized measures have been developed. They include: Biochemical Oxygen Demand (BOD), which is the quantity of oxygen in milligrams per liter or parts per million utilized in the biochemical oxidation of the organic matter contained in the water within a period of 5 days at 20.degree. C. and often designated as BOD.sub.5 ; and Chemical Oxygen Demand (COD), which is the quantity of oxygen expressed in milligrams per liter or parts per million consumed under specific oxidation conditions with strong chemical oxidizing agents, such as sodium chromate (see Standard Methods for the Examination of Water and Waste Water, 12th Edition, Public Health Association, New York, N.Y., (1965), pp. 510-514, which is incorporated herein by reference). Generally, the acceptable minimal standard expressed in terms of BOD.sub.5 and COD for a purified waste water stream is about 20 and about 100 milligrams per liter, respectively.
Recent standards being promulgated for pollution abatement are generally stated in terms emphasizing COD rather than BOD. Municipal sewerage generally has a BOD.sub.5 prior to entry into the primary settler discussed above ranging from about 100 to about 150 milligrams per liter. Municipalities which have a significant amount of industrial waste water discharged into their sewerage system may have a BOD.sub.5 within the range of 200 to 400 milligrams per liter or higher. After the municipal sewerage has been treated in the primary settling step, the BOD.sub.5 will be in the range from about 50 to about 100 milligrams per liter or higher if a substantial amount of industrial waste is contained in the municipal sewerage. The COD will generally be considerably higher than the BOD.sub.5 depending upon the amount of nonbiodegradable material in the sewerage. For example, if the municipal sewerage contains primarily sanitary sewerage, the COD will be only slightly higher than the BOD.sub.5. However, with a substantial quantity of effluents from industrial plants contained in the sewerage, the COD may be two to three times as high as the BOD.sub.5, both before and after the sewerage has been passed through the primary settling step. After the secondary biological treatment, the BOD.sub.5 as well as the COD from the purely sanitary sewerage, will be normally in the range of from about 20 to about 35 milligrams per liter. However, in the event a substantial amount of industrial waste is included in the municipal sewerage, the effluent from the secondary biological treatment can exhibit a COD of 100 milligrams per liter or higher.
Accordingly, it can be seen that the removal of nonbiodegradable impurities from industrial waste waters requires treatment additional to that necessary for normal sanitary sewerage. This is due primarily to the industrial waste waters containing a much higher concentration of nonbiodegradable impurities than is present in normal sanitary sewerage. For example, the COD of such industrial waste waters can range from about 100 to 2000 milligrams per liter and in some cases as high as 5000 to 6000 milligrams per liter. Since large amounts of nonbiodegradable organic compounds are present in the industrial waste waters after the biological secondary treatment, the effluent from said treatment may still have a COD as high as 600 to 1000 milligrams per liter. Furthermore, industrial waste waters usually have a high concentration of toxic materials such that biological treatment, even under the best of conditions, is generally unreliable and subject to frequent destruction of the biological organism. Hence, the effluents passed to receiving waters from such treatment can approach the COD of the raw waste waters. This is particularly true with regard to bio-resistant contaminants such as aromatics, halogenated hydrocarbons, nitrated hydrocarbons, and tertiary alcohols, which are characteristic of petroleum refining and organic chemical manufacturing waste waters. Although these organic contaminants may be nonbiodegradable and thus not considered to deplete the oxygen content of the receiving waters, they may biodegrade over a period greater than the five days measured by the BOD test and thus deplete oxygen in larger rivers and lakes. Furthermore, these bio-resistant contaminants may be noxious since such contaminants affect the taste, odor and color of the receiving waters and exhibit toxic effects on the fish and plant life therein. Thus, even when the biolgical treatment plants are operating under optimum conditions, the amount of organic contaminants removed may not be sufficient to meet the standards presently being established. As a consequence, there is a need for further treating of the effluents from such biological secondary treatment plants, as well as a need for an improved process for treating industrial waste waters, in order to remove both biodegradable and nonbiodegradable organic contaminants therefrom so as to prevent the undersirable results mentioned above.
In order to remove the organic contaminants from waste waters, particularly industrial waste waters, it has been proposed to treat the waste waters as well as the effluent from the secondary biological treatment step with activated carbon. For example, U.S. Pat. Nos. 3,244,621, 3,455,820 and 3,658,697 disclose methods for removing organic soluble impurities from waste water by passing said waste water through a bed of activated carbon. However, it has also been disclosed (see Hopkins, C. B., Weber, W. J., Jr., Bloom, R., Jr., U.S. Department of interior Federal Water Pollution Control Administration Report No. TWRC-2, Dec. 1968) that during the prolonged treatment of municipal and industrial waste waters with granular activated carbon, there occurs a significant buildup of sludge in the carbon bed(s) due to biological activity in the beds. The production of this biological sludge, in addition to presenting a sludge removal and disposal problem, results in plugging of the carbon beds, thereby requiring frequent backwashing in order to obtain satisfactory hydraulic operation of the beds. In addition, anaerobic biological activity occurring in the activated carbon bed also results in the undesirable generation of hydrogen sulfide which is discharged from the carbon bed (see U.S. Pat. No. 3,658,697).
Methods of employing activated carbon to remove contaminants from waste water have often met with limited commercial success due to either the limited adsorption capability or the high cost of the adsorbents. For example, when delayed coke is employed as the adsorbent, only suspended oils and phenolic compounds are removed from the waste water stream. The use of activated carbon has been severely inhibited by associated processing difficulties and the inherent high initial cost of the material. In addition, the high attrition and regeneration losses which occur when activated carbon is employed in the prior art processes mentioned above result in high make-up costs. Activated carbons in powdered form, although available at relatively low initial cost since they are produced largely by the partial incineration of waste liquid from paper manufacture, are difficult to remove from the treated water because of their highly subdivided state which results in very low settling rates. Thus, when using powdered activated carbon, each contact stage requires a subsequent settling stage having a long residence time and the use of expensive organic polymers as flocculents. Furthermore, after removal from the water, no practical techniques have been developed for regenerating activated powdered carbon for reuse. Thus, even if the initial unit cost of the powdered activated carbon is relatively low, the overall operating cost becomes exorbitant since the material can be used only once and then must be disposed of at an additional cost.
Granular activated carbons, such as those produced from coal, are expensive adsorbents because they require a multi-step process for their manufacture in order to produce them with uniform particle size and acceptable hardness. Even though these materials have greater hardness and attrition resistance than the so-called "soft" activated carbons produced from other materials such as wood, nut shells, and the like, the attrition resistance from granular activated carbon still leaves much to be desired. Specifically, a significant portion of the granular activated carbon is lost due to attrition in the handling and use of the material. This may occur, for example, when the granular activated carbon is removed from the waste water contacting bed(s) and regenerated in a device such as a multiple-hearth furnace, the regenerated granular activated carbon then being recycled to the contacting bed(s). Not only does this represent a high operating cost due to the make-up with fresh granular activated carbon material, but the fines produced by said attrition are difficult to remove from the treated water and, therefore, represent a source of contamination.
Accordingly, since activated carbon is a comparatively expensive material for use in removing contaminants from waste water streams, there is a need for a method of increasing the contaminant removal capacity of the activated carbon material.